Texture Painting on image is not showing up in Blender 2.8
The reason that the colour is not showing up is because your image is being obscured by the UV mesh of your model which is crazy high resolution. In order to disable it go to View and uncheck "Display Texture Paint UVs":


The reason your whites are showing up as grays is because of how light works, your model gets shaded and will only display at full brightness under very strong light (which won't show in deafult Viewport Shading mode). It's normal.
In order to see the true colours on your mesh you can select flat shading for your viewport:


Also I would highly advise disabling your subsurf modifier for your texture painting, my God that donut mesh lags out my pretty decant laptop. I don't know what kind of PC you have but I'm almost sure you'll get a better expirience without the subsurf on.